Search for rare events in [the square root of sigma nu nu] = 200 GeV Au+Au PHOBOS data

Abstract

In this analysis, we set an upper bound on the rate of rare events: events whose dN/dn distribution deviates more than statistically from the ensemble average dN/dn distribution for s [the square root of sigma nu nu] = 200 GeV Au+Au collisions in PHOBOS data. We carefully remove events that may exhibit non-statistical fluctuations due to other effects, such as event pileup and detector pathology. We also use very fine binning in the z and y vertex positions to eliminate fluctuations due to different event vertices. We eliminate global correlations within the dN/dn distributions by using a covariance matrix in the analysis, which is used to scale out correlations between difference pseduorapidity (n) regions. In the end we produce a value of x2 per degree of freedom (X2v ) for each event, which reflects how well the event agrees with the ensemble average. We compare this distribution with the distribution we would expect for a model using uncorrelated random variables with the same degrees of freedom of our system, and find remarkable agreement between our data and this random distribution. This allows us to conclude that most events in our data set are statistical fluctuations about the ensemble average. We are further able to determine that there is a signal of non-statistical events with X2V > 2.22, and that the rate of these events in the PHOBOS data is less than (1.97 ± 0.4(stat) O0.1(sys)) x 10-5 rare events/events.